Carbamate esters, also known as urethanes, have a variety of uses, such as in the production of polyurethanes, isocyanates, and pesticides. These esters are generally prepared by: (1) reacting with an amine a chloroformate ester that is obtained by reacting phosgene with an alcohol; or (2) by reacting an isocyanate with an alcohol. U.S. Pat. No. 5,556,971 discloses a process for preparing isocyanate and isocyanate-based 1,3,5-triazine derivatives by the direct phosphogenation of an at least tris-unsubstituted amino-1,3,5-triazine. PCT publication WO 96/11915 discloses a process for preparing acid amides, including isocyanate-functional 1,3,5-triazines and isocyanate-based 1,3,5-triazine derivatives, from the reaction of silicon, germanium or tin substituted amino-1,3,5-triazines and acid halides. Both phosgene and isocyanates are considered as undesirable reactants due to their toxicity.
Many derivatives of amino-1,3,5-triazines are utilized in a wide variety of applications, as described in the literature. One important use of certain of these derivatives, such as methoxymethyl derivatives of melamine and guanamines, is as crosslinkers in curable compositions. Although these triazine derivatives are useful for such applications, formaldehyde may be undesirably released as a volatile by-product during curing. As a result, there is much interest in finding alternative 1,3,5-triazine derivatives that do not emit formaldehyde upon curing.
One such alternative are the alkoxycarbonylamino-1,3,5-triazines, also known as triazine carbamate esters. U.S. Pat. No. 5,288,865 discloses a one step process for the preparation of carbamate functional 1,3,5-triazines by reacting a haloamino-1,3,5-triazine with an acid halide.
U.S. Pat. Nos. 4,939,213 and 5,084,541 each disclose a process for preparing 1,3,5-triazine carbamates in two steps. Both steps involve reacting an amino-1,3,5-triazine with oxalyl chloride to produce an isocyanate intermediate, then further reacting the intermediate with an alcohol. The resulting 1,3,5-triazine carbamates are potentially useful crosslinkers in coating compositions that are based upon hydroxy functional resins. There are disadvantages to this process, including the required use of certain costly halogenated starting materials, the production of significant amounts of halogenated by-products, and the low overall yield of the desired products.
Replacing oxalyl chloride with carbon monoxide in the carbonylation step solves the problems associated with halogenated reactants and products. Thus, U.S. Pat. Nos. 3,405,156 and 3,641,092 disclose processes for carbonylation of amines in the presence of a catalyst to form isocyanates. It is generally known that carbamates may be formed by subsequent reaction of isocyanates with an alcohol to produce the corresponding carbamate ester, and this was further shown in both cases.
U.S. Pat. No. 4,694,097 discloses a process for producing carbamate esters by reacting an amine with carbon monoxide and an alcohol in the presence of a protonic acid, a dehydrating agent, and a catalyst. The protonic acid and the dehydrating agent are reported to be key components in this process.
Similarly, European Patent Application No. 173,457 discloses a process for producing aromatic carbamate esters from aromatic amines, carbon monoxide, an alcohol, and a protonic acid in the presence of a catalyst.
U.S. Pat. No. 5,194,660 discloses a process for producing carbamates by reacting primary amines, secondary amines, and/or ureas with carbon monoxide, an organic hydroxyl compound, an oxidizing agent, a halide, and a catalytic metal macrocyclic complex.
European Patent Application No. 649,842 discloses a process for producing carbamates by the carbonylation of triazines. The process involves the reaction of a triazine with carbon monoxide and an alcohol in the presence of a metal catalyst system and an oxidant system. In the specific case of reacting a tris-amino-1,3,5-triazine, mono- and bis-carbamate triazines were formed, but small amounts (e.g., less than 6%) of the tris-carbamate triazine was formed.
European Patent Application No. 624,577 and U.S. Pat. Nos. 5,705,641 and 5,792,866 disclose a process for producing bis-carbamate functional 1,3,5-triazines by reacting diamino-1,3,5-triazines with an acyclic organic carbonate and a base. Yields of between 30 to 80% tris-carbamate triazines were reportedly obtained.
It is also generally known that carbamates may be obtained via the carbonylation of amines in the presence of an alcohol. However, as it is well known to the skilled person in the art that the amine functionality of amino 1,3,5-triazines (such as amine functionalities of melamines and guanamines) is not equivalent to the other types of typical amine functionality, one of ordinary skill in the art would not reasonably expect success when conducting multiple carbonylations on tris-amino-1,3,5-triazines.
Furthermore, melamines and guanamines are among the least reactive of the amines and, thus, the most difficult to functionalize. Their behavior is not normally correlated to that of other known amines, even structurally similar amines such as pyrimidines. For example, most typical amines are highly reactive with acyl halides. Smolin el al. reported unsuccessful attempts to acylate all three amino groups of melamine (E. M. Smolin and L. Rappaport, "S-Triazines and Derivatives", Interscience Publishers Inc., New York, 1959, pg 333).
There is thus a need for a process for producing tris-carbamates that can function as environmentally friendly crosslinkers by not emitting formaldehyde upon curing. To date, no process of efficiently producing tris-carbamates by carbon monoxide have been reported. The present invention now satisfies this need.